Metals low in impurities are desirable for applications where product uniformity, metallurgical homogeneity and high fatigue resistance are required, such as in aerospace applications. Metallic components are often formed and shaped from metallic powders; for example, composite materials can be manufactured by sintering a mixture of metallic powders.
Small, spherical metallic particles are presently produced in the Rotating Electrode Process in which an elongate, high purity rotatable rod or disk is inserted into an atmospherically controlled chamber and confronted with a stationary electrode. The rod serves as the second electrode; an arc struck between the electrodes consumes the rod or disk material as it rotates. The rotation casts off melted material which solidifies into powder. This procedure is costly, however, when it is necessary to accurately machine the rod serving as the electrode; the rod typically rotates at 15,000 rpm and therefore must be extremely uniform in dimension.
Those nickel-based alloys which are commonly referred to as Superalloys present difficulties for the manufacture of pure, uniform powders using the rotating electrode process. The alloys themselves are presently manufactured in batch processes in which appropriate quantities of blocks or bars of each constituent metal are melted and mixed together and are cast into rod form. The resulting rods have typical cast structures with dendrites formed as the alloy solidifies. The dendrites involve short-range variations of chemical composition in the microstructure of the solidified rod and this deviation from homogeneity may be transmitted to powders made from the rod. When dendrites are in the rod serving as the rotating electrode dendrite parts may become present in varying amounts in the molten metal droplets which are spun off by centrifugal force; variations in chemical composition between powder particles ultimately results.
Other processes seek to avoid dendrite formation by working directly with molten metal. In the gas atomization process, a gas of high molecular weight such as argon is directed at a stream of molten metal. Impurities are introduced by contact of the liquid metal with the refractory ceramic linings of crucibles and ladles used to melt and handle the molten metal. Skull melting avoids pick-up of crucible materials but cannot avoid formation and entrainment of dendrite arms and other inhomogeneous particles.
Magnetic levitation is utilized in several processes to transport molten metal. The Dynapour made by Ajax Magnathermic consists of a trough including induction coils which are used to draw liquid metal against gravity over the lip of a crucible and direct the liquid metal into a mold or diecasting machine. Significantly, the liquid metal may come into physical contact with the trough during transport. The contact can cause problems since some metals are more compatible with lining materials than others. For example, liquid copper is compatible with graphite. Titanium, molybdenum, and other metals which form carbides with graphite actively corrode the graphite and therefore are much more difficult to maintain in pure form.
Lowry et al., U.S. Pat. No. 4,414,285, disclose a continuous, contactless metal casting system which raises at a controlled rate molten metal introduced in a liquid state at the lower end of a casting column. A gap is maintained between the molten metal and the inner surface of the column but the gap is minimized to enhance heat transfer between the molten metal and a cooling jacket surrounding the casting column. Significantly, a solid in the form of a continuous rod is removed from the levitation column as the final product. The shape of the form is determined to some extent by the shape of the magnetic field.
The rate at which metals solidify can affect their microcrystalline structure which in turn produces metals having unique properties. Amorphous, ribbon-shaped metals are presently formed by quenching a thin stream of liquid metal against a hollow metal drum chilled by water. The cooling rate required for sufficiently rapid solidification may be as high as 10.sup.8 .degree.C./sec. Contact of the liquid metal with the cooling drum rarely introduces impurities since the solidification is so rapid. However, conventional delivery of the liquid metal to the drum may introduce undesirable impurities to the metal.